1. Field of the Invention
The present invention relates to a toner, a method of manufacturing the toner, a developing device, and an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus which employs an electrophotographic system, image formation is accomplished by forming a toner image through development of an electrostatic latent image formed on a photoreceptor with the supply of toner and then fixing the toner image onto a recording medium. A toner for use in such an image forming apparatus is produced by blending, in a binder resin, raw materials such as a colorant, release agent, and a charge control agent and then granulating the mixture so as to obtain a predetermined particle size.
Used toner is discarded by means of soil burial or incineration. However, the disposal of used toner by incineration results in emission of carbon dioxide, which is one of greenhouse gases, into the air. Furthermore, there is a possibility that metal substances contained in a colorant, a charge control agent, and so forth become the source of environmental pollutant. Thus, there have been proposed a large number of toners that can be discarded while preventing environmental contamination.
For example, in Japanese Unexamined Patent Publication JP-A 4-218063 (1992) is disclosed a toner containing at least a binder resin, a colorant, a charge control agent, and a biodegradable resin, and also a toner containing a photodecomposition agent. When the toner disclosed in JP-A 4-218063 (1992) is discarded by means of soil burial, by virtue of the inclusion of a biodegradable resin, the toner can be decomposed while preventing environmental contamination. However, the negative side is that the biodegradable resin exhibits poor crushability and thus the microparticulation therefor is hard to achieve. This makes it difficult to produce a toner of small particle size required for forming a high-quality, high-resolution image.
In order to solve such a problem, in Japanese Unexamined Patent Publication JP-A 2004-177554 is disclosed a toner manufacturing method that involves a step of preparing a coloring solution by dissolving or dispersing a biodegradable resin and a colorant in an organic solvent and a step of mixing the coloring solution and an aqueous medium to form coloring resin fine particles. In the case of producing a toner by the method disclosed in JP-A 2004-177554, even if the biodegradable resin has poor crushability, fine particles can be obtained with ease. This makes it possible to produce a toner of small particle size.
FIG. 8 is a sectional view of a toner particle 51 of related art. According to the toner disclosed in JP-A 4-218063, the binder resin and the biodegradable resin are mixed in a molten or softened state and thereafter the mixture is cooled down. In this case, the biodegradable resin is crystallized and is thus dispersed in a sea-island state within the toner particle. That is, in the toner disclosed in JP-A 4-218063, as shown in FIG. 8, in the toner particle 51, namely a sea component, biodegradable resin 52-made island components of varying size are scattered in an unstable state where their shapes cannot be identified on an individual basis.
Such a toner is susceptible to toner cracking which occurs at the interface between the binder resin and the biodegradable resin. This makes it impossible for the biodegradable resin to be contained in the toner particle at a high percentage. Furthermore, the crystallized portions of the biodegradable resin vary in size from small to large and are thus dispersed in an intricately-shaped state, which results in a decline in toner transparency. As a result, in the case of applying such a toner to a color toner, the range of color reproduction is narrowed. In addition, the biodegradable resin is not uniformly dispersed and thus the toner becomes uneven in composition, which gives rise to lack of uniformity in the characteristics of the individual toner particles. This makes it impossible to control toner properties such as charging characteristics.
According to the toner manufacturing method disclosed in JP-A 2004-177554, the toner is obtained by melting or softening the biodegradable resin in an organic solvent and subjecting it to phase inversion emulsification in an aqueous medium. Since such a toner contains the biodegradable resin as a binder resin, it follows that crystallization takes place due to the heat generated at the time of fixing to a recording medium, which results in a decline in transparency. In the case of applying such a toner to a color toner, the range of color reproduction is narrowed. Furthermore, such a toner is low in durability.